1. Field of the Invention
The present invention relates generally to gel compounds within conduits or buffer tubes and more specifically to the reduction of dripping of the gel compounds at higher temperatures while providing additional protection to optical fibers from water penetration.
2. Description of Related Art
Fiber optic cables have been used by the telecommunications industry for a number of years to transmit information at very high rates over long distances. Fiber optic cables come in a variety of configurations, including: cables with a centrally located single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside.
The buffer tubes within the ribbon cable generally contain one or more fiber optic ribbons centrally located within the buffer tube and a gel compound surrounding the optical fiber ribbons. An example of this can be seen in FIGS. 1-4. As shown in these figures, the fiber optic ribbons 3 are centrally located within buffer tube 1. As can be further seen from FIGS. 1-4, a gel compound 2 surrounds the fiber optic ribbons 3. The gel compound 2 serves a number of purposes. One purpose is to provide a cushioning media between the buffer tube 1 and the fiber optic ribbons 3 to thereby prevent the fiber optic ribbons 3 from contacting the sides of the buffer tube 1. The cushioning media dissipates radial crushing force and in addition, the gel compound 2 provides delayed motion response to the fibers under scanning bending loads. Such loads occur during the installation, when cables are pulled around the corners of the ducts or over the sheaves. The same applies to the earlier stages of manufacture when buffer tube 1 is bent over the sheaves and radially compressed by caterpillars. The artificial increase in the inertia of the ribbons 3 is provided by the viscous gel media and results in time delay for fibers to accommodate the load and to move slower than in a non-gel media toward the tube walls 1. When the fiber optic ribbons 3 contact the sides of the buffer tube 1, signal attenuation problems occur due to micro-bending and high stress. The gel compound 2 also serves to prevent exterior items from coming into contact with the fiber optic ribbons 3 if the buffer tube 1 is penetrated. For example, the gel compound 2 protects the fiber optic ribbons 3 from water that might penetrate the buffer tube 1.
Several problems occur in these conventional buffer tubes, especially ones in which the buffer tube 1 diameter is large (for example, greater than 0.310 inches). First, when the temperature of the gel compound 2 increases, the viscosity and yield stress of the gel compound 2 decreases. If the yield stress of the gel compound 2 decreases below a critical value, the gel compound 2 may begin to flow. For example, if the buffer tube 1 is physically positioned in a vertical manner, as shown in FIG. 5, and the buffer tube 1 is heated, the gel compound 2 within the buffer tube 1 may begin to flow towards the bottom of the buffer tube 1, leaving a cavity 4.
In more detail, as the temperature of the buffer tube 1 increases, the buffer tube 1 expands, thereby increasing the diameter and length of the buffer tube 1, according to the difference between the coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) of the buffer tube material 1 and gel compound 2. As for the gel compound 2, as noted above, as its temperature increases, the viscosity and yield stress of the gel compound 2 decreases. As shown in FIG. 5, gravity provides a downward force to the gel compound 2 while frictional forces (F1 and F2) with the tube wall are transmitted through the material by the yield stress of the gel compound 2. Friction between the gel compound 2 and the buffer tube 1 is labeled F1 while the fiction between the gel compound 2 and the fiber optic ribbons 3 is labeled F2. Consequently, as the temperature of the gel compound 2 increases, the yield stress of the gel compound 2 decreases and the ability of the gel to transmit friction forces F1 and F1 through the gel compound 2 decreases. Since the downward force of gravity remains constant during an increase in temperature of the gel compound 2, when the temperature of the gel compound 2 increases, the downward force of gravity on the material becomes greater than the upward force that can be transmitted through the material through the yield stress of the gel compound 2. As a result, the gel compound 2 may flow downward.
Once the gel compound 2 xe2x80x9cruns away,xe2x80x9d it does not provide adequate protection to the fiber optic ribbons 3. The fiber optic ribbons 3 tend to contact the buffer tube walls 1, which in turn causes attenuation problems. Therefore, it is an object of the present invention to improve the compound flow performance of gel compound-filled fiber optic cables.
Additionally, gel compound 2 may be xe2x80x9cforcedxe2x80x9d out of the buffer tube 1 when heated due to the difference between the CTE of the buffer tube 1 and the CTE of the gel compound 2. As stated earlier, when heated, both the buffer tube 1 and the gel compound 2 expand according to their respective CTE. If the CTE of the buffer tube 1 is less than the CTE of the gel compound 2, then the gel compound 2 expands more than the buffer tube 1. Since the gel compound 2 is expansionally limited in the radial direction by the buffer tube 1, if the gel compound 2 expands more than the buffer tube 1 when heated, the additional expansion of the gel compound 2 is directed in the axial direction. As a result, gel compound 2 is xe2x80x9cforcedxe2x80x9d out of the ends of the buffer tube 1.
Another problem occurs when the gel compound-filled buffer tubes 1 contain relatively large air bubbles 6. The air bubbles 6 are often formed by the upward movement and coalescence of smaller air bubbles that were not completely removed from the gel compound 2 under vacuum conditions. Additionally, air bubbles 6 can be entrapped in the gel compound 2 during the extrusion of the gel compound 2 and buffer tube 1. In horizontal position, coalesced bubbles 6 form continuous air passages. Air bubbles 6 can significantly ease water penetration and can further help water, that has penetrated the buffer tube 1, to move axially within the buffer tube 1. Exposing optical fiber 3 to water can result in optical fiber deterioration.
Another problem with the conventional buffer tubes is the stability or integrity of the ribbon stack shape under the bending loads. In particular, when the stack twist laylength is long and the tube or cable is bent about a small-radius object, the ribbon stack may collapse. The ribbons 3 may slide sideways and xe2x80x9cfallxe2x80x9d sideways causing ribbon damage and fiber attenuation. Thus, it is desirable to provide means for holding the stack with initial, typically rectangular shape of its cross section.
According to one aspect of the invention, optical fibers are provided in a conduit along with water swellable material and a gel compound. In one of the preferred embodiments, the conduit is a buffer tube.
More specifically, the present invention solves the above-described problems and limitations by placing water swellable yarns and/or water absorbing particles within gel compound-filled conduits or the buffer tubes. The water swellable yarns and particles, when in contact with water that has penetrated the buffer tube, begin to swell. As a result, the yarns and particles provide many beneficial effects. First, when water penetrates the buffer tube and comes in contact with the water swellable yarns, the yarns volumetrically expand and fill out the volume taken by the air bubble, as well as break air channels in the horizontally positioned portions of the tubes. Second, the yarns help to compensate for the expansion of the buffer tube walls caused by the increase in temperature by making the expansion and CTE of the gel-yarn system closer to that of the buffer tube material. Finally, the yarns themselves provide surface area for the gel compound to contact which in turn provides additional friction forces that help to keep the gel compound from xe2x80x9cflowingxe2x80x9d downward.
In a preferred embodiment of the present invention, water swellable yams having whiskers (increased total surface area) are disposed between the fiber optic ribbon stacks and the walls of the buffer tube. The water swellable yarns vary in size and surround the fiber optic ribbon stacks.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.